Ion source



W. T. LELAND Jan. 5, 1960 ION SOURCE Filed Aug. 19, 1955 INVEN TOR.

BY Wallace 7'. Leland KM/ZfiJ-J ATTORNEY United States of America as represented by the United States Atomic Energy Commission Applicafim A?list Serial No. 529,593 1 Claim. (Cl. 250-413 The present invention relates to mass spectrometry, and more especially to an improved ion source incorporating a novel filament-trap assembly for forming ions in a mass spectrometer. The improved source is especially adapted for long continuous periodsof operation when the gases analyzed are corrosive or tend to stick to the surfaces of the sou'rce. 4 t

It is wellknown that a gas may be bombarded by a stream of electrons and that a certain number of its molecules will become ionized thereby, If a gaseous mixture is similarly treated, a mixture'of ions of various kinds will result. Such a mixture of ions may be subjected to the action of a combination of electrostatic and magneticfields to cause those ions having a particular mass to impinge upon a collecting member to which electrons will flow, thus creating an electric current. This current may be amplified and used as a measure of the number of ions having the said particularmass. A mass spectrometer is a device adapted to analyze the components of a neutral gaseous mixture fed intoit by bombarding such mixture with a stream of electrons, accelerating the resulting ions by passing them through a drop in electrical potential, and directing them into a magnetic field where ions having the same mass are bent to follow the same path. Then due to the influence of the magnetic field, the ions divide themselves into groups, each group containing only ,ions ofthe same mass and each group following a different path. Ions of any particular path may be projected upon and made to strike a collector, and the resulting current flowfthereto serves as a measure of the quantity of such ions. The mass spectrometer maybe adjusted by changing the accelerating potential to collect ions having. a given mass, and the number of the ions having this mass is proportional to the molecules from which the ions were produced. Therefore, it is entirely feasible to employ the mass spectrometer as a gasanalyzingmeans. v

The usefulness of mass spectrometers has heretofore been impaired by certain inherent defects in their construction, particularly in the arrangement of an ion source where the gases are ionized by electrons emitted from a filament. The intensity of the ion currents produced have been limited and the spectrometers have become insensitive after short periods of operation due to the formation of non-conductive deposits 'onelements of the source, most particularly on the anode, which is subjected to continuous electron bombardment. Especially when certain corrosive gases are analyzed in a mass spectrometer, the formation of non-volatile, nonconducting deposits is accelerated very rapidly on several surfaces which are exposed to electron bombardment, so that the spectrometer becomes insensitive and virtually useless after only a relatively short exposure. Thenrin order to restore the sensitivity of the instrument, the tedious procedure of dismantling, cleaning, and reassembling the instrument is necessary. Furthermore, shutdown of the instrument for servicing results in the nonnitcd States Patent k 2,920,200 l atented Jan. 5, 969

availability of the equipment for rather frequent, undesirably long periods.

An evenmore serious defect, because it is more insidious, is the slow but sure change in sensitivity resulting as the deposits build up within the source, so that the user of instruments of the prior art is never sure of the sensitivity 'at any particular time, or how long. a run he will be able to make before the instrumentfails. Impurities deposit on the anode and form an insulating sheath over it, causing the electron beam to shift to a clean spot. The beam in shifting causes instability in the ionizing region, and often shifts so far as to strike the chamber walls themselves. Obviously, such defects make prior instruments unsatisfactory for continuous process control.

With the knowledge of the very serious defects and limitations which limit the usefulness of the mass spectrometer in analysis of certain corrosive gases and those which tend to stick to the surfaces, applicant has as an object of his invention provision of a novel source which is suitable for long, continued operation at optimum sensitivity.

Another object of the invention is to provide a novel ion source such that the rate of formation of troublesome deposits. thereon is substantially reduced. Yet another object of the invention is to provide an ion sourceincluding a filament, an anode, means for focusing electrons from the filament to the anode, and separate means for continuously supplying an electrical heating current through the anode duringoperation of the spectrometer, thereby substantially inhibiting the formation of the undesired layers of material in the electron P th- Still another object 'of the invention is to provide a novel source geometry whereby the condensation of nonconductign materials from thermal decomposition on both the trap and the filament is inhibited.

These and other objects of the present invention and a manner in which these objects are achieved will be better understood from the following detailed description of a preferred embodiment of the invention, when read in conjunction with the appended drawings, in which:

Figure 1 is a diagrammatic view of a conventional mass spectrometer with which the improved ion source may be employed;

Figure 2 is a fragmental sectional view of a spectrometer incorporating a conventional ion source and taken along the line 22 of Figure 1, but in a plane displaced angularly ninety degrees from the showing of Figure l, and

Figure 3 is a longitudinal sectional elevation of a preferred form of an improved, source as it may be installed in the spectrometer of Figures 1 and 2, in the same orientation as the source of Figure 2.

According to the invention, a filament is provided outside the ionization chamber of an ion source and preferably, but not necessarily farther removed therefrom than in the conventional source construction. An electron trap or anode is provided also outside the ionization chamber and also may preferably be at a greater distance therefrom than in the conventional source structure. Means are provided for heating the trap during normal operation of the spectrometer, the trap being maintained at a higher temperature than the ionization chamber itself. A focusing electrode is provided outside the ion chamber to focus substantially all the electrons emitted by the filament through slits in the ion chamber walls and onto the heater trap in applications where a strong collimating magnetic field is not present. By means of this novel source arrangement, substantially all the electrons are focused onto the trap, so that none of the other source parts are subject to electron bombardment, and therefore the undesirable deposits do not build up on these surfaces. Yet by continuously passing a heating current through the trap during operation, electron collection is accomplished but the insulating deposits are inhibited; hence the beam does not shift, the instrument is stabilized, and also the life of the source is greatly increased. 7

Referring now to Figure 1, a conventional spectrometer such as may incorporate the invention is shown. Such a spectrometer is described in my prior Patent 2,643,341 issued June 23, 1953. It is to be understood that the present invention is equally suitable for use in other types of spectrometers, such as the all-metal type, the present description being purely by way of illustration. In the spectrometer, the ion beam travels through an evacuated tube 1, which may be of copper tubing. I At one end of the tube is a glass envelope 2 having disposed therewitbin an ion source 3 which is mounted on the tube. 1. Various leads 4 pass through the envelope 2 and connect to the internal source elements in a conventional manner.

The source 3 includes an elongated enclosure generally designated 5 which extends transversely of the envelope 2 and defines the ionization chamber 10. The enclosure 5 is shown in detail in Figure 3, to which reference is now made, and is defined by an upper plate 16, lower plate 20, one pair of opposite elongated spaced walls 14,15, and another pair of opposite front and back walls. In communication with an opening in the back wall 13 is a tube 12 which passes out through the envelope 2 and-serves as a sample tube for supplying neutral gases to the ionizing chamber for analysis. Mounted outside of chamber 10 on parallel leads 18 is a V-shaped filament 19. The filament bridges the lower extremities of the leads 18, 18, which are supported by insulating sleeve 22 mounted on bracket 21, which is in turn supported by rod 23. The rod is mounted to the case 24 and supported by bracket 25, and carries the focussing electrode 26 near its lower extremity. In the vertical section of the bracket 15, in alignment with the filament and a collimat- Ling slot 27 in the opposite wall 14 of the enclosure 5 is a slot 9.

Disposed external to the chamber 10 and in alignment with the filament and slots is a trap or anode 30 supported by two rods 31 housed in glass insulator 32, which is mounted on bracket 33, rod 34, and brace member 35. The rods 31 connect to the moveable arm 36 and a fixed point 37 of a potentiometer, which is coupled across the secondary of a filament transformer 38. The primary of the transformer may be supplied with alternating current from a source 39. Positioned beneath the lower wall of plate of the ionizing chamber 10 are a series of electrically isolated spaced plates 21, 22, 23, 24 and 25. Plate 21 is split into two halves as clearly shown in my prior patent, above mentioned. These plates may be held in spaced relation by means of insulating spacers, two of which are designated 40 and 41, carried by bolts 42, 43 which thread the various spacers and extend through appropriately dimensioned openings in the plates so as to be spaced and insulated therefrom. Nuts may be applied to the bolts 42, 43, and the whole structure may be.

held in assembled relation in the manner common to the conventional spectrometer source.

Positioned externally of the envelope 2 on either side of the source and aligned with the electron flow may be the poles 49, 50 of a magnet for setting up a magnetic field along the ionizing chamber, in the conventional manner. Insulating sleeves such as those shown in Figure 4 of my patent, supra, may serve to house the leads running to plates of the ion source, in the conventional manner, but are here omitted for clarity of illustration.

The trap may be made from a one mil tungsten ribbon, 80 mils wide, for example, and is preferably heated to its normal operating dull-red heat temperature by passing alternating current through it from an external source. Satisfactory operation is achieved with a 2.5 mil tungsten ribbon, inch wide, the dimensions of the trap being 4 not of a critical nature. To derive the heating current, the source may be connected, for example, to the primary of a common filament transformer, the secondary wind ing of which is shunted by a potentiometer. The moveable arm on the potentiometer may be used to select the amount of heating current to Vary the temperature of the trap during operation.

In operation of one embodiment of the invention, the case may be operated at ground potential, with the filament 75 volts negative with respect to the case and the trap operated 75 volts positive with respect to the case. The filament may be energized by a source of filament current, preferably providing substantially 5-6 amperes at .5-1 volts. The trap may be heated from a source of volts alternating current, with the exact amount of heating being regulated through the potentiometer to control the heating of the, trap as desired. It is desired for long trap life, but not critical in operation, to operate the trap at the lowest possible temperature consistent with proper operation, and it has been found in practice that these conditions are satisfied when the trap is just visibly red in color. The electrodes constituted by the two halves of plate 21 may be operated 110 volts negative with respect to the case 20, electrode 22 may be volts negative, electrode 23 may be at ground potential or slightly above, in the order of .6 volt, while the outer electrodes 24, 25 may be operated at ground potential, in the manner disclosed in my prior patent, supra.

In order to minimize the effects of thermal decomposition at the filament and the heated trap, the distances between these heated bodies and the ionization chamber proper may be increased over those common to the prior art chambers. It has been found that with increased spacing, the decomposition product will deposit mainly outside the ionization chamber in regions where the nonconducting surface layers have little or no efiect on source operation. In normal spacing, the filament and trap may be .25 inch from the chamber walls. The spacing may be adjusted as convenient, and .5 inch spacing from the walls has been utilized for both filament and trap.

To better focus the electron stream on the heated trap, a focusing electrode has been provided between filament and the sidewell of the chamber. This electrode may take the form of a cylinder inch in length and .175 inch internal diameter into one end of which the filament protrudes. The cylinder is preferably electrically isolated from the remainder of the source and provision made for its potential to be varied to obtain a slight amount of focusing. In practice, it has been found satisfactory to connect the cylinder to the case 24 to obtain the focusing potential.

When the increased spacing between filament, trap, and chamber is utilized, a glass housing and source magnet slightly larger than those provided with the spectrometer shown in the reference patent are provided. The housing provided is similar to the conventional one except that it has a larger diameter to accommodate the wider spacing between source elements. In any event it has extra lead-entry means, such as S-press seals, for receiving the added leads required for heating the trap and for energizing the extra electrode. A larger source magnet corresponding to the larger housing may be obtained by merely substituting a larger yoke with the same poles for the previously used magnets, as described in my prior patent, supra.

Thus it will be apparent to those skilled in the art that a novel ion source for mass spectrometers has been provided which greatly increases the useful life of the source and hence helps make the instrument suitable for continuous process control. Moreover, with the improved source, highly corrosive samples and also other gases which heretofore formed deposits on the source parts and soon rendered the source in the machine insensitive can now be analyzed with accuracy.

Having described the invention, what is claimed as novel is:

An improved ion source for mass spectrometers comprising an envelope defining an ionization chamber, said envelope being provided with first and second apertured side walls and ion entrance and exit passageways; a filament mounted outside said envelope, in spaced relation to said first wall, and aligned with the apertures in said side walls; a cylindrical focussing electrode disposed between said filament and said first wall to focus electrons from said filament through said apertures in said side walls; an anode mounted outside said envelope, in spaced relation to said second wall, and aligned with said apertures to receive said electrons; and means for heating said anode to a temperature higher than that within said ionization chamber.

References Cited in the file of this patent UNITED STATES PATENTS 

